PHP Constants :- 

• A constant is a name or an identifier for a simple value.
• A constant value cannot change during the execution of the script.
• By default, a constant is case-sensitive.
• Constant identifiers are always uppercase.
• A constant name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.
• If we have defined a constant, it can never be changed or undefined.
• To define a constant you have to use define() function.
• To retrieve the value of a constant, we have to simply specifying its name.

constant() function :-

• By its name, we can understand this function will return the value of the constant.
• This is useful when we want to retrieve value of a constant, but we do not know its name.
• It is stored in a variable or returned by a function.
• Only scalar data like boolean, integer, float and string can be contained in constants.

Differences between variables and constants :- 

• Constants cannot be defined by simple assignment, they may only be defined using the define() function.
• Constants may be defined and accessed anywhere without regard to variable scoping rules.
• Once the Constants have been set, may not be redefined or undefined.
• There is no need to write a dollar sign ($) before a constant, where as in Variable one has to write a dollar sign.